1. Field of the Invention
The present invention relates generally to positioning systems and methods, and more specifically to positioning systems and methods which control the movement of a substrate such as a semiconductor wafer which includes numerous integrated circuits located on the substrate.
2. Background Information
Positioning systems are used in different areas of semiconductor fabrication. For example, wafer probers, photolithographic systems and other semiconductor integrated circuit fabrication devices often use a wafer positioning system. Most wafer positioning systems are based on combinations of linear motions of one or more stages along orthogonal directions. An example of a conventional linear motion wafer positioning system in a wafer prober will now be described with reference to FIGS. 1 and 2.
A semiconductor wafer usually includes numerous integrated circuits arranged in a grid pattern. Located on each integrated circuit (IC) is a plurality of bonding pads that are used to connect the IC to external circuitry to allow the IC to function. Because the packaging of each IC is rather expensive, it is desirable to test each IC before packaging to avoid packaging defective IC""s. This process of testing devices before packaging is called the sort process, and it also involves connecting a probe card to a special tester. FIG. 1 depicts a prior art wafer prober system 10 that includes a basic example of a probe card 16 mounted on a support 15. Probe card 16 includes a number of pins 17 that act as substitutes for the normal pins and wire leads of a package device. Pins 17 are made to come into electrical contact with the bonding pads 13 of at least one integrated circuit on a semiconductor wafer 12 that rests on a wafer chuck 11, which is also called a wafer holding platform. Wafer prober system 10 positions each IC on the wafer with respect to probe card 16 so that the appropriate pins 17 on probe card 16 contact the appropriate bonding pads 13 for a particular IC on semiconductor wafer 12.
As the art of semiconductor processing advances, semiconductor wafers become larger, die geometries become smaller, the number of pads on each die increases, and the size of each pad decreases. Thus, the alignment accuracy and speed requirements for a wafer prober become more stringent, placing great demands on the positioning stages used in a wafer prober. The positioning stages are aided by modern vision systems that use cameras, such as cameras 14 and 19 that are designed to view probe card 16 and semiconductor wafer 12, respectively, to attempt to accurately align an IC on a semiconductor wafer with respect to the pins on a probe card.
FIG. 2 depicts a conventional prior art wafer positioning system that can be used in a wafer prober or other semiconductor fabrication device. When used in wafer probers, positioning systems must provide four axes (X,Y,Z,xcex8) of motion. A common implementation includes an X,Y stage for positioning in X,Y and an independent Z to stage and an independent xcex8 stage. FIG. 2 shows a wafer positioning system 20 using a conventional rectilinear X,Y stage. A wafer chuck 25 is positioned on top of a Y-motion stage 21 which moves along guide rails 22a and 22b to provide translation in the Y axis. Wafer chuck 25 may include a rotary motor located on top of Y motion stage 21 and below wafer chuck 25 to provide xcex8 motion for wafer chuck 25. An X-motion stage 23 moves along guide rails 24a and 24b to provide translation in the X axis. A semiconductor wafer 30 is positioned on top of wafer chuck 25 and is typically held in place by a vacuum generated under the surface of the semiconductor wafer by the wafer chuck. A separate Z stage (not shown) provides translation in the Z axis by either changing the distance between wafer chuck 25 and Y-motion stage 21 or by moving both X-motion stage 23 and Y-motion stage 21 along with their corresponding guide rails in the Z axis.
FIG. 3A shows a side view of a simplified representation of a prior art wafer positioning system that uses linear motors, such as Sawyer motors. A wafer holding stage 40 comprises electromagnetic assemblies 37 bonded to each other by a permanent magnet 38; the coupling of two electromagnetic assemblies and a permanent magnet forms a motor. The motor is bonded to a material layer 39. Wafer holding stage 40 is located above and separated from a platform 35 by a thin air gap 36.
A limitation of prior art wafer positioning systems that are based on 82 combinations of linear motions along orthogonal directions is that the wafer is moved on along only one direction by one set of motors or one stage. For example, with reference to FIG. 2, semiconductor wafer 30 is moved in the X direction by only X-motion stage 23, and it is moved in the Y direction by only Y-motion stage 21. Thus, at any time the wafer is being moved, one stage is left unused. As speed requirements for wafer positioning systems increase, it is desirable to provide a wafer positioning system that utilizes multiple avenues of movement concurrently and works within current physical constraints of wafer positioning systems.
The present invention provides a wafer positioning system which includes in one embodiment a motor system to move a wafer holding stage and a motion control system to control a path of the wafer holding stage. The motor system constrains the wafer holding stage to a first set of two movement axes, which are the physical axes, and the motion control system controls the path of the wafer holding stage along a second set of two movement axes, which are the non-physical axes. The first set of movement axes comprise axes at an angle to each other, and the second set of movement axes also comprise axes at an angle to each other. The first and second sets of axes are also located at an angle to each other. The distance a wafer on the wafer holding stage is to be moved is measured along one of the non-physical axes. To move the wafer along this distance, the motion control system moves the wafer holding stage along the physical axes of the motor system concurrently.
The present invention also provides a method for moving a wafer holding stage in a wafer positioning system. In one example of this method, the movement of the wafer holding stage is constrained to a first and a second axis, and the path of the wafer holding stage is controlled along a third and a fourth axis. In another example of this method, the step of controlling the path of the wafer holding stage includes the steps of measuring along the third or fourth axes the distance a wafer is to be moved and moving the wafer holding stage along the first and second axes concurrently.
Additional features and benefits of the present invention will become apparent upon review of the following description.